Solder mask inkjet inks for manufacturing printed circuit boards

ABSTRACT

The invention provides an inkjet method for producing a solder mask in the manufacture of a Printed Circuit Board. By using a solder mask inkjet ink containing at least one photoinitiator, at least one free radical polymerizable compound and at least one mercapto functionalized carboxylic acid as adhesion promoter, a high quality solder mask withstanding the high thermal stress during the soldering process while maintaining excellent physical properties, may be produced.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of application Ser. No.16/348,163, filed May 8, 2019, which is a 371 National Stage Applicationof PCT/EP2017/078390, filed Nov. 7, 2017, which claims the benefit ofEuropean Application No. 16198090.9, filed Nov. 10, 2016, the contentsof which are incorporated by reference herein in their entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a solder mask inkjet ink and an inkjetmethod for manufacturing Printed Circuit Boards.

BACKGROUND ART FOR THE INVENTION

Inkjet printing methods have been proposed to further improve themanufacturing process of Printed Circuit Boards (PCBs).

Inkjet printing methods and inkjet inks have been disclosed for legendprinting in for example EP-A 2725075 (Agfa) and for applying an etchresist on a copper surface in for example EP-A 2809735 (Agfa) and EP-A3000853 (Agfa).

By reducing the complexity and minimizing the waste such inkjet printingmethods render the manufacture of PCBs more cost effective.

Also for applying the solder mask, inkjet printing methods have beendisclosed in for example EP-A 1543704 (Avecia) and EP-A 1624001 (TaiyoInk Manufacturing).

Solder masks are permanent protective coatings that perform a number offunctions during the fabrication, assembly and end use of PCBs. One ofthe main purposes of solder mask is to protect the circuitry frominteracting with solder during the assembly process. A solder mask's jobisn't solely restricted to the solder operation however, as it alsohelps to protect the laminate, holes and traces from collectingcontaminants and from degrading during the service life of the PCB. Asolder mask also acts as an insulator of known dielectric propertybetween components and traces of the PCB.

UV curable inks are preferred for the design of solder mask inks as theyallow a high crosslinking degree, resulting in excellent chemicalresistance and mechanical properties. However, compatibility with thehigh temperature soldering process, while maintaining all physicalproperties, is especially challenging.

The solder mask inkjet inks disclosed in EP-A 1513704 include anadhesion promoter, which is preferably a (meth)acrylate containing anacid such as for example 2-carboxyethylacrylate.

In EP-A 1624001 the solder mask inkjet ink comprises a monomer having a(meth)acryloyl group and a thermosetting functional group selected fromthe group consisting of a hydroxyl group, a carboxyl group, anisocyanate group, an amino group, an imino group, an epoxy group, anoxetanyl group, a mercapto group, a methoxymethyl group, a methoxyethylgroup, an ethoxymethyl group, an ethoxyethyl group, and an oxazolinegroup.

There is still a need for designing solder mask inkjet inks, which areable to withstand the high thermal stress induced during the solderingprocess in the manufacture of PCBs.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a solder mask inkjet inkwherewith a high quality solder mask, in particular withstanding thehigh thermal stress during the soldering process while maintainingexcellent physical properties, may be produced.

The object of the invention is realized by the solder mask inkjet inkaccording to claim 1.

Further objects of the invention will become apparent from thedescription hereinafter.

DETAILED DESCRIPTION OF THE INVENTION Definitions

The term “monofunctional” in e.g. monofunctional polymerizable compoundmeans that the polymerizable compound includes one polymerizable group.

The term “difunctional” in e.g. difunctional polymerizable compoundmeans that the polymerizable compound includes two polymerizable groups.

The term “polyfunctional” in e.g. polyfunctional polymerizable compoundmeans that the polymerizable compound includes more than twopolymerizable groups.

The term “alkyl” means all variants possible for each number of carbonatoms in the alkyl group i.e. methyl, ethyl, for three carbon atoms:n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl andtertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl,2,2-dimethylpropyl and 2-methyl-butyl, etc.

Unless otherwise specified a substituted or unsubstituted alkyl group ispreferably a C₁ to C₆-alkyl group.

Unless otherwise specified a substituted or unsubstituted alkenyl groupis preferably a C₂ to C₆-alkenyl group.

Unless otherwise specified a substituted or unsubstituted alkynyl groupis preferably a C₂ to C₆-alkynyl group.

Unless otherwise specified a substituted or unsubstituted aralkyl groupis preferably a phenyl or naphthyl group including one, two, three ormore C₁ to C₆-alkyl groups.

Unless otherwise specified a substituted or unsubstituted alkaryl groupis preferably a C₇ to C₂₀-alkyl group including a phenyl group ornaphthyl group.

Unless otherwise specified a substituted or unsubstituted aryl group ispreferably a phenyl group or naphthyl group.

Unless otherwise specified a substituted or unsubstituted heteroarylgroup is preferably a five- or six-membered ring substituted by one, twoor three oxygen atoms, nitrogen atoms, sulphur atoms, selenium atoms orcombinations thereof.

The term “substituted”, in e.g. substituted alkyl group means that thealkyl group may be substituted by other atoms than the atoms normallypresent in such a group, i.e. carbon and hydrogen. For example, asubstituted alkyl group may include a halogen atom or a thiol group. Anunsubstituted alkyl group contains only carbon and hydrogen atoms.

Unless otherwise specified a substituted alkyl group, a substitutedalkenyl group, a substituted alkynyl group, a substituted aralkyl group,a substituted alkaryl group, a substituted aryl and a substitutedheteroaryl group are preferably substituted by one or more constituentsselected from the group consisting of methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl and tertiary-butyl, ester, amide, ether,thioether, ketone, aldehyde, sulfoxide, sulfone, sulfonate ester,sulphonamide, —Cl, —Br, —I, —OH, —SH, —CN and —NO₂.

Manufacture of an Electronic Device

A method of manufacturing an electronic device according to the presentinvention includes the steps of:

-   -   jetting a radiation curable solder mask inkjet ink as described        below on a dielectric substrate containing an electrically        conductive pattern; and    -   curing the jetted solder mask inkjet ink.

The electronic device is preferably a Printed Circuit Board.

The radiation curable solder mask inkjet ink may be cured by exposingthe ink to actinic radiation, such as electron beam or ultraviolet (UV)radiation.

Preferably the radiation curable inkjet ink is cured by UV radiation,more preferably using UV LED curing.

The method preferably includes a thermal treatment. The thermaltreatment is preferably carried out after the curing step.

In a preferred embodiment the thermal treatment is carried out at atemperature from 80° C. to 250° C. The temperature is preferably notless than 100° C., more preferably not less than 120° C. To preventcharring of the solder mask, the temperature is preferably not greaterthan 200° C., more preferably not greater then 160° C.

The thermal treatment is typically carried out between 15 and 90minutes.

The purpose of the thermal treatment is to further increase thepolymerization degree of the solder mask.

This further polymerization during the thermal treatment may beaccelerated by adding radical initiators which promote thermal curing ofpolymers, such as peroxides and azo compounds, to the solder mask inkjetink.

The dielectric substrate of the electronic device may be anynon-conductive material. The substrate is typically a paper/resincomposite or a resin/fibre glass composite, a ceramic substrate, apolyester or a polyimide.

The electrically conductive pattern is typically made from any metal oralloy which is conventionally used for preparing electronic devices suchas gold, silver, palladium, nickel/gold, nickel, tin, tin/lead,aluminium, tin/aluminium and copper. The electrically conductive patternis preferably made from copper.

Radiation Curable Solder Mask Inkjet Ink

The radiation curable solder mask inkjet ink comprises at least onephotoinitiator, at least one free radical polymerizable compound, and atleast one adhesion promoter as disclosed below.

The solder mask inkjet ink can be cured by e-beam, but is preferablycured by UV light, more preferably by UV light from UV LEDs. The soldermask inkjet ink is thus preferably a UV curable inkjet ink.

For reliable industrial inkjet printing, the viscosity of the radiationcurable inkjet inks is preferably no more than 20 mPa·s at 45° C., morepreferably between 1 and 18 mPa·s at 45° C., and most preferably between4 and 14 mPa·s at 45° C.

For good image quality and adhesion, the surface tension of theradiation curable inkjet inks is preferably in the range of 18 to 70mN/m at 25° C., more preferably in the range of 20 to 40 mN/m at 25° C.

The radiation curable inkjet ink may further comprise otherpolymerizable compounds, colorants, polymeric dispersants, apolymerization inhibitor, or a surfactant.

Adhesion Promoter

The adhesion promoter has a chemical formula according to Formula I,

wherein

L₁ represents an optionally substituted n+m-valent linking groupcomprising 1 to 15 carbon atoms,

n and m independently represent an integer from 1 to 4.

Preferably n equals 1 and m represents 1 or 2.

L₁ preferably comprises 1 to 10, more preferably 1 to 8 carbon atoms.

L₁ preferably is selected from the group consisting of an optionallysubstituted alkylene group and an optionally substituted arylene group.

In a preferred embodiment the adhesion promoter has a chemical formulaaccording to Formula II,

wherein

L₂ represents an optionally substituted o+p-valent linking groupcomprising 1 to 10 carbon atoms,

o and p independently represent an integer from 1 to 4, Q represents thenecessary atoms to form an optionally substituted 5 or 6 memberedheteroaromatic ring.

L₂ preferably comprises 1 to 8, more preferably 1 to 6 carbon atoms.

Q preferably represents the necessary atoms to form an optionallysubstituted 5 membered heteroaromatic ring selected from the groupconsisting of a 1,2,3-triazole, a 1,2,4-triazole, a 1,3,4-oxadiazole, a1,3,4-thiadiazole, a tetrazole, an imidazole, a benzimidazole, anoxazole, a benzoxazole, a thiazole and a benzthiazole.

In a particularly preferred embodiment the adhesion promoter has achemical structure according to Formula III,

wherein

L₃ represents a q+r-valent linking group comprising 1 to 10 carbonatoms, q and r independently represent an integer from 1 to 4.

Preferably, q equals 1 and r represents 1 or 2.

L₃ preferably comprises 1 to 8, more preferably 1 to 6 carbon atoms.

Typical examples of adhesion promoters according to the presentinvention are given in Table 1 without being limited thereto.

TABLE 1

Thio-1

Thio-2

Thio-3

Thio-4

Thio-5

Thio-6

Thio-7

Thio-8

Thio-9

Thio-10

Thio-11

Thio-12

Thio-13

Thio-14

Thio15

The radiation curable inkjet may in addition to the adhesion promoterdescribed above comprise other adhesion promoters, for examples thosedisclosed in WO2004/028225.

The amount of adhesion promoter in the radiation curable solder maskinkjet ink is preferably between 0.5 and 20 wt %, more preferablybetween 1 and 15 wt %, most preferably between 2.5 and 10 wt %, relativeto the total weight of the inkjet ink.

Free Radically Polymerizable Compounds

The free radical polymerizable compounds may be monomers, oligomersand/or prepolymers.

These monomers, oligomers and/or prepolymers may possess differentdegrees of functionality. A mixture including combinations of mono-,di-, tri- and higher functional monomers, oligomers and/or prepolymersmay be used. The viscosity of the radiation curable inkjet ink may beadjusted by varying the ratio between the monomers and oligomers.

In a preferred embodiment, the monomer, oligomer or polymer includes atleast one acrylate group as polymerizable group.

Preferred monomers and oligomers are those listed in paragraph [0106] to[0115] in EP-A 1911814.

In a preferred embodiment, the radiation curable solder mask inkjet inkcomprises a monomer containing a vinylether group and an acrylate ormethacrylate group. Such monomers are disclosed in EP-A 2848659,paragraphs [0099] to [0104]). A particular preferred monomer containinga vinylether group and an acrylate group is2-(2-vinyloxyethoxy)ethylacrylate.

In a particular preferred embodiment, the radiation curable solder maskinkjet ink comprises a free radical polymerizable compound selected fromthe group consisting of neopentyl-glycol hydroxypivalate diacrylate,isobornyl acrylate, dipropylene glycol diacrylate, trimethylol propanetriacrylate, and 2-(vinylethoxy)ethyl acrylate.

Colorants

The radiation curable solder mask inkjet ink may be a substantiallycolourless inkjet ink, but preferably the inkjet ink includes at leastone colorant.

The colorant in the solder mask inkjet ink may be a pigment or a dye,but is preferably a pigment.

A colour pigment may be chosen from those disclosed by HERBST, Willy, etal.

Industrial Organic Pigments, Production, Properties, Applications, 3rdedition. Wiley-VCH, 2004, ISBN3527305769.

Suitable pigments are disclosed in paragraphs [0128] to [0138] ofWO2008/074548. Pigment particles in inkjet inks should be sufficientlysmall to permit free flow of the ink through the inkjet-printing device,especially at the ejecting nozzles. It is also desirable to use smallparticles for maximum colour strength and to slow down sedimentation.Most preferably, the average pigment particle size is no larger than 150nm. The average particle size of pigment particles is preferablydetermined with a Brookhaven Instruments Particle Sizer B190 plus basedupon the principle of dynamic light scattering.

In PCBs, the solder mask typically has a blue or green colour. The bluepigment is preferably one of the phthalocyanine series. Examples of bluepigments are C.I. Pigment Blue 1, 15, 15:1, 15:2, 15:3, 15:4, 15:6, 16,24 and 60.

Green pigments are generally a mixture of blue and yellow or orangepigments or may be green pigments per se, such as halogenatedphthalocyanines, for example copper or nickel brominated phthalocyanine.

In a preferred embodiment, the colorant is present in an amount of 0.2to 6.0 wt %, more preferably 0.5 to 2.5 wt %, based on the total weightof the radiation curable inkjet ink.

Polymeric Dispersants

If the colorant in the radiation curable inkjet is a pigment, then theradiation curable inkjet preferably contains a dispersant, morepreferably a polymeric dispersant, for dispersing the pigment.

Suitable polymeric dispersants are copolymers of two monomers but theymay contain three, four, five or even more monomers. The properties ofpolymeric dispersants depend on both the nature of the monomers andtheir distribution in the polymer. Copolymeric dispersants preferablyhave the following polymer compositions:

-   -   statistically polymerized monomers (e.g. monomers A and B        polymerized into ABBAABAB);    -   alternating polymerized monomers (e.g. monomers A and B        polymerized into ABABABAB);    -   gradient (tapered) polymerized monomers (e.g. monomers A and B        polymerized into AAABAABBABBB);    -   block copolymers (e.g. monomers A and B polymerized into        AAAAABBBBBB) wherein the block length of each of the blocks (2,        3, 4, 5 or even more) is important for the dispersion capability        of the polymeric dispersant;    -   graft copolymers (graft copolymers consist of a polymeric        backbone with polymeric side chains attached to the backbone);        and    -   mixed forms of these polymers, e.g. blocky gradient copolymers.

Suitable polymeric dispersants are listed in the section on“Dispersants”, more specifically [0064] to [0070] and [0074] to [0077],in EP-A 1911814.

Commercial examples of polymeric dispersants are the following:

-   -   DISPERBYK™ dispersants available from BYK CHEMIE GMBH;    -   SOLSPERSE™ dispersants available from NOVEON;    -   TEGO™ DISPERS™ dispersants from EVONIK;    -   EDAPLAN™ dispersants from MÜNZING CHEMIE;    -   ETHACRYL™ dispersants from LYONDELL;    -   GANEX™ dispersants from ISP;    -   DISPEX™ and EFKA™ dispersants from CIBA SPECIALTY CHEMICALS INC;    -   DISPONER™ dispersants from DEUCHEM; and    -   JONCRYL™ dispersants from JOHNSON POLYMER.        Photoinitiators and Photoinitiating Systems

The radiation curable solder mask inkjet ink preferably contains atleast one photoinitiator, but may contain a photoinitiating systemincluding a plurality of photoinitiators and/or co-initiators.

The photoinitiator in the radiation curable inkjet is preferably a freeradical initiator, more specifically a Norrish type I initiator or aNorrish type II initiator. A free radical photoinitiator is a chemicalcompound that initiates polymerization of monomers and oligomers whenexposed to actinic radiation by the formation of a free radical. ANorrish Type I initiator is an initiator which cleaves after excitation,yielding the initiating radical immediately. A Norrish type II-initiatoris a photoinitiator which is activated by actinic radiation and formsfree radicals by hydrogen abstraction from a second compound thatbecomes the actual initiating free radical. This second compound iscalled a polymerization synergist or co-initiator. Both type I and typeII photoinitiators can be used in the present invention, alone or incombination.

Suitable photoinitiators are disclosed in CRIVELLO, J. V., et al.Photoinitiators for Free Radical Cationic and AnionicPhotopolymerization. 2nd edition. Edited by BRADLEY, G. London, UK: JohnWiley and Sons Ltd, 1998. p. 287-294.

Specific examples of photoinitiators may include, but are not limitedto, the following compounds or combinations thereof: benzophenone andsubstituted benzophenones, 1-hydroxycyclohexyl phenyl ketone,thioxanthones such as isopropylthioxanthone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-benzyl-2-dimethylamino-(4-morpholinophenyl) butan-1-one, benzyldimethylketal, bis (2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphineoxide, 2,4,6 trimethylbenzoyl-diphenylphosphine oxide,2,4,6-trimethoxybenzoyldiphenylphosphine oxide,2-methyl-1-[4-(methylthio) phenyl]-2-morpholinopropan-1-one,2,2-dimethoxy-1,2-diphenylethan-1-one or 5,7-diiodo-3-butoxy-6-fluorone.

Suitable commercial photoinitiators include Irgacure™ 184, Irgacure™500, Irgacure™ 369, Irgacure™ 1700, Irgacure™ 651, Irgacure™ 819,Irgacure™ 1000, Irgacure™ 1300, Irgacure™ 1870, Darocur™ 1173, Darocur™2959, Darocur™ 4265 and Darocur™ ITX available from CIBA SPECIALTYCHEMICALS, Lucerin™ TPO available from BASF AG, Esacure™ KT046, Esacure™KIP150, Esacure™ KT37 and Esacure™ EDB available from LAMBERTI, H-Nu™470 and H-Nu™ 470X available from SPECTRA GROUP Ltd.

The photoinitiator may be a so-called diffusion hindered photoinitiator.A diffusion hindered photoinitiator is a photoinitiator which exhibits amuch lower mobility in a cured ink layer than a monofunctionalphotoinitiator, such as benzophenone. Several methods can be used tolower the mobility of the photoinitiator. One way is to increase themolecular weight of the photoinitiators so that the diffusion speed isreduced, e.g. polymeric photoinitiators. Another way is to increase itsreactivity so that it is built into the polymerizing network, e.g.multifunctional photoinitiators (having 2, 3 or more photoinitiatinggroups) and polymerizable photoinitiators.

The diffusion hindered photoinitiator for the radiation curable inkjetis preferably selected from the group consisting of non-polymericmultifunctional photoinitiators, oligomeric or polymeric photoinitiatorsand polymerizable photoinitiators. Most preferably the diffusionhindered photoinitiator is a polymerizable initiator or a polymericphotoinitiator.

A preferred diffusion hindered photoinitiator contains one or morephotoinitiating functional groups derived from a Norrish typeI-photoinitiator selected from the group consisting of benzoinethers,benzil ketals, α,α-dialkoxyacetophenones, α-hydroxyalkylphenones,α-aminoalkylphenones, acylphosphine oxides, acylphosphine sulphides,α-haloketones, α-halosulfones and phenylglyoxalates.

A preferred diffusion hindered photoinitiator contains one or morephotoinitiating functional groups derived from a Norrish typeII-initiator selected from the group consisting of benzophenones,thioxanthones, 1,2-diketones and anthraquinones.

Suitable diffusion hindered photoinitiators are also those disclosed inEP-A 2065362 in paragraphs [0074] and [0075] for difunctional andmultifunctional photoinitiators, in paragraphs [0077] to [0080] forpolymeric photoinitiators and in paragraphs [0081] to [0083] forpolymerizable photoinitiators.

A preferred amount of photoinitiator is 0.1-20 wt %, more preferably2-15 wt %, and most preferably 3-10 wt % of the total weight of theradiation curable inkjet.

In order to increase the photosensitivity further, the radiation curableinkjet may additionally contain co-initiators. Suitable examples ofco-initiators can be categorized in three groups: 1) tertiary aliphaticamines such as methyldiethanolamine, dimethylethanolamine,triethanolamine, triethylamine and N-methylmorpholine; (2) aromaticamines such as amylparadimethyl-aminobenzoate,2-n-butoxyethyl-4-(dimethylamino) benzoate,2-(dimethylamino)-ethylbenzoate, ethyl-4-(dimethylamino)benzoate, and2-ethylhexyl-4-(dimethylamino)benzoate; and (3) (meth)acrylated aminessuch as dialkylamino alkyl(meth)acrylates (e.g.,diethylaminoethylacrylate) or N-morpholinoalkyl-(meth)acrylates (e.g.,N-morpholinoethyl-acrylate). The preferred co-initiators areaminobenzoates.

When one or more co-initiators are included into the radiation curableinkjet ink, preferably these co-initiators are diffusion hindered forsafety reasons.

A diffusion hindered co-initiator is preferably selected from the groupconsisting of non-polymeric di-or multifunctional co-initiators,oligomeric or polymeric co-initiators and polymerizable co-initiators.More preferably the diffusion hindered co-initiator is selected from thegroup consisting of polymeric co-initiators and polymerizableco-initiators. Most preferably the diffusion hindered co-initiator is apolymerizable co-initiator having at least one (meth)acrylate group,more preferably having at least one acrylate group.

The radiation curable inkjet ink preferably includes a polymerizable orpolymeric tertiary amine co-initiator.

Preferred diffusion hindered co-initiators are the polymerizableco-initiators disclosed in EP-A 2053101 in paragraphs [0088] and [0097].

The radiation curable solder mask inkjet ink preferably includes the(diffusion hindered) co-initiator in an amount of 0.1 to 20 wt %, morepreferably in an amount of 0.5 to 15 wt %, most preferably in an amountof 1 to 10 wt % of the total weight of the radiation curable inkjet ink.

Polymerization Inhibitors

The radiation curable solder mask inkjet ink may contain at least oneinhibitor for improving the thermal stability of the ink.

Suitable polymerization inhibitors include phenol type antioxidants,hindered amine light stabilizers, phosphor type antioxidants,hydroquinone monomethyl ether commonly used in (meth)acrylate monomers,and hydroquinone, t-butylcatechol, pyrogallol,2,6-di-tert.butyl-4-methylphenol (=BHT) may also be used.

Suitable commercial inhibitors are, for example, Sumilizer™ GA-80,Sumilizer™ GM and Sumilizer™ GS produced by Sumitomo Chemical Co. Ltd.;Genorad™ 16, Genorad™18 and Genorad™ 20 from Rahn AG; Irgastab™UV10 andIrgastab™ UV22, Tinuvin™ 460 and CGS20 from Ciba Specialty Chemicals;Floorstab™ UV range (UV-1, UV-2, UV-5 and UV-8) from Kromachem Ltd,Additol™ S range (S100, S110, S120 and S130) from Cytec SurfaceSpecialties.

The inhibitor is preferably a polymerizable inhibitor.

Since excessive addition of these polymerization inhibitors may lowerthe curing speed, it is preferred that the amount capable of preventingpolymerization is determined prior to blending. The amount of apolymerization inhibitor is preferably lower than 5 wt %, morepreferably lower than 3 wt % of the total radiation curable inkjet ink.

Surfactants

The radiation curable solder mask inkjet ink may contain at least onesurfactant, but preferably no surfactant is present. If no surfactant ispresent, the radiation curable inkjet ink does not spread well allowingthe generation of thin lines.

The surfactant can be anionic, cationic, non-ionic, or zwitter-ionic andis usually added in a total quantity less than 1 wt % based on the totalweight of the radiation curable inkjet ink.

Suitable surfactants include fluorinated surfactants, fatty acid salts,ester salts of a higher alcohol, alkylbenzene sulfonate salts,sulfosuccinate ester salts and phosphate ester salts of a higher alcohol(for example, sodium dodecylbenzenesulfonate and sodiumdioctylsulfosuccinate), ethylene oxide adducts of a higher alcohol,ethylene oxide adducts of an alkylphenol, ethylene oxide adducts of apolyhydric alcohol fatty acid ester, and acetylene glycol and ethyleneoxide adducts thereof (for example, polyoxyethylene nonylphenyl ether,and SURFYNOL™ 104, 104H, 440, 465 and TG available from AIR PRODUCTS &CHEMICALS INC.).

Preferred surfactants are selected from fluoric surfactants (such asfluorinated hydrocarbons) and silicone surfactants. The siliconesurfactants are preferably siloxanes and can be alkoxylated, polyethermodified, polyether modified hydroxy functional, amine modified, epoxymodified and other modifications or combinations thereof. Preferredsiloxanes are polymeric, for example polydimethylsiloxanes.

Preferred commercial silicone surfactants include BYK™ 333 and BYK™UV3510 from BYK Chemie.

In a preferred embodiment, the surfactant is a polymerizable compound.

Preferred polymerizable silicone surfactants include a (meth)acrylatedsilicone surfactant. Most preferably the (meth)acrylated siliconesurfactant is an acrylated silicone surfactant, because acrylates aremore reactive than methacrylates.

In a preferred embodiment, the (meth)acrylated silicone surfactant is apolyether modified (meth)acrylated polydimethylsiloxane or a polyestermodified (meth)acrylated polydimethylsiloxane.

Preferably the surfactant is present in the radiation curable inkjet inkin an amount of 0 to 3 wt % based on the total weight of the radiationcurable inkjet ink.

Flame Retardant

The radiation curable solder mask inkjet ink may further contain a flameretardant. In principle all known flame retardants may be used. However,the flame retardant is preferably not a halogen containing compound.

Preferred flame retardants are inorganic flame retardants, such asAlumina Trihydrate and Boehmite, and organo-phosphor compounds, such asorgano-phosphates (e.g. triphenyl phosphate (TPP), resorcinol bis(diphenylphosphate) (RDP), bisphenol A diphenyl phosphate (BADP), andtricresyl phosphate (TCP)); organo-phosphonates (e.g. dimethylmethylphosphonate (DMMP)); and organo-phosphinates (e.g. aluminiumdiethyl phosphinate).

Other preferred organo-phosphor compounds are disclosed in U.S. Pat. No.8,273,805.

Preparation of Inkjet Inks

The preparation of pigmented radiation curable inkjet inks is well-knownto the skilled person. Preferred methods of preparation are disclosed inparagraphs [0076] to [0085] of WO2011/069943.

Inkjet Printing Devices

The radiation curable solder mask inkjet ink may be jetted by one ormore print heads ejecting small droplets in a controlled manner throughnozzles onto a substrate, which is moving relative to the print head(s).

A preferred print head for the inkjet printing system is a piezoelectrichead. Piezoelectric inkjet printing is based on the movement of apiezoelectric ceramic transducer when a voltage is applied thereto. Theapplication of a voltage changes the shape of the piezoelectric ceramictransducer in the print head creating a void, which is then filled withink. When the voltage is again removed, the ceramic expands to itsoriginal shape, ejecting a drop of ink from the print head. However theinkjet printing method according to the present invention is notrestricted to piezoelectric inkjet printing. Other inkjet print headscan be used and include various types, such as a continuous type.

The inkjet print head normally scans back and forth in a transversaldirection across the moving ink-receiver surface. Often the inkjet printhead does not print on the way back. Bi-directional printing ispreferred for obtaining a high areal throughput. Another preferredprinting method is by a “single pass printing process”, which can beperformed by using page wide inkjet print heads or multiple staggeredinkjet print heads which cover the entire width of the ink-receiversurface. In a single pass printing process the inkjet print headsusually remain stationary and the ink-receiver surface is transportedunder the inkjet print heads.

Curing Devices

The radiation curable solder mask inkjet ink can be cured by exposingthem to actinic radiation, such as electron beam or ultravioletradiation. Preferably the radiation curable inkjet ink is cured byultraviolet radiation, more preferably using UV LED curing.

In inkjet printing, the curing means may be arranged in combination withthe print head of the inkjet printer, travelling therewith so that thecurable liquid is exposed to curing radiation very shortly after beenjetted.

In such an arrangement, with the exception of UV LEDs, it can bedifficult to provide a small enough radiation source connected to andtravelling with the print head.

Therefore, a static fixed radiation source may be employed, e.g. asource of curing UV-light, connected to the radiation source by means offlexible radiation conductive means such as a fibre optic bundle or aninternally reflective flexible tube.

Alternatively, the actinic radiation may be supplied from a fixed sourceto the radiation head by an arrangement of mirrors including a mirrorupon the radiation head.

The source of radiation may also be an elongated radiation sourceextending transversely across the substrate to be cured. It may beadjacent the transverse path of the print head so that the subsequentrows of images formed by the print head are passed, stepwise orcontinually, beneath that radiation source.

Any ultraviolet light source, as long as part of the emitted light canbe absorbed by the photo-initiator or photo-initiator system, may beemployed as a radiation source, such as, a high or low pressure mercurylamp, a cold cathode tube, a black light, an ultraviolet LED, anultraviolet laser, and a flash light. Of these, the preferred source isone exhibiting a relatively long wavelength UV-contribution having adominant wavelength of 300-400 nm. Specifically, a UV-A light source ispreferred due to the reduced light scattering therewith resulting inmore efficient interior curing.

-   -   UV radiation is generally classed as UV-A, UV-B, and UV-C as        follows:    -   UV-A: 400 nm to 320 nm    -   UV-B: 320 nm to 290 nm    -   UV-C: 290 nm to 100 nm.

In a preferred embodiment, the radiation curable inkjet ink is cured byUV LEDs. The inkjet printing device preferably contains one or more UVLEDs preferably with a wavelength larger than 360 nm, preferably one ormore UV LEDs with a wavelength larger than 380 nm, and most preferablyUV LEDs with a wavelength of about 395 nm.

Furthermore, it is possible to cure the ink image using, consecutivelyor simultaneously, two light sources of differing wavelength orilluminance. For example, the first UV-source can be selected to be richin UV-C, in particular in the range of 260 nm-200 nm. The secondUV-source can then be rich in UV-A, e.g. a gallium-doped lamp, or adifferent lamp high in both UV-A and UV-B. The use of two UV-sources hasbeen found to have advantages e.g. a fast curing speed and a high curingdegree.

For facilitating curing, the inkjet printing device often includes oneor more oxygen depletion units. The oxygen depletion units place ablanket of nitrogen or other relatively inert gas (e.g. CO₂), withadjustable position and adjustable inert gas concentration, in order toreduce the oxygen concentration in the curing environment. Residualoxygen levels are usually maintained as low as 200 ppm, but aregenerally in the range of 200 ppm to 1200 ppm.

EXAMPLES

Materials

All materials used in the following examples were readily available fromstandard sources such as ALDRICH CHEMICAL Co. (Belgium) and ACROS(Belgium) unless otherwise specified. The water used was deionizedwater.

SR606A is neopentylglycol hydroxypivalate diacrylate available asSartomer™ SR606A from ARKEMA.

ITX is Speedcure™ ITX, a mixture of isopropyl thioxanthone isomers, fromLAMBSON SPECIALTY CHEMICALS.

TPO is 2,4,6-trimethylbenzoyldiphenylphosphine oxide, supplied by RAHNAG

EPD is ethyl-4-(dimethylamino)benzoate, available under the trade nameof Genocure™ EPD from RAHN AG.

INHIB is a mixture forming a polymerization inhibitor having acomposition according to Table 2.

TABLE 2 Component wt % DPGDA 82.4 p-methoxyphenol 4.02,6-di-tert-butyl-4-methylphenol 10.0 Cupferron ™ AL 3.6

Cupferron™ AL is aluminum N-nitrosophenylhydroxylamine from WAKOCHEMICALS LTD.

Ebecryl 1360 AK is a polysiloxane hexa acrylate slip agent from ALLNEX.

DPGDA is dipropylenediacrylate, available as Sartomer SR508 from ARKEMA.

TMPTA is trimethylol propane triacrylate, available as Sartomer™ SR351from ARKEMA.

VEEA is 2-(vinylethoxy)ethyl acrylate available from NIPPON SHOKUBAI,Japan.

Cyan is SUN FAST BLUE 15:4, a cyan pigment available from SUN CHEMICALS.

Yellow is CROMOPHTAL YELLOW D 1085J, a yellow pigment from BASF.

Disperbyk 162 is a dispersing agent and has been precipitated from asolution available from BYK (ALTANA).

THIO-3 is carboxy-3-sulfanyl propionic acid from ALDRICH.

THIO-6 is thiosalicylic acid from ACROS.

THIO-9 is N-acetyl-L-cysteine from ALDRICH.

Methods

Coatings/Prints of the Solder Mask Inkjet Inks

To evaluate the adhesion and the solder resistance of the solder maskinkjet inks, the inks were coated on a brushed copper foil (35μ) at acoating thickness of 20μ and cured using a H-bulb (1 pass at 20 m/min).Additionally, the coatings were thermally cured at 150° C. during 30minutes.

Evaluation Solder Resistance

The solder resistance of the solder mask inkjet inks was evaluated usinga SPL600240 Digital Dynamic Solder Pot available from L&M PRODUCTSfilled with a “K” Grade 63:37 tin/lead solder available from SOLDERCONNECTION. The temperature of the solder was set at 290° C.

Using a Q-tip, a solder flux SC7560A from SOLDER CONNECTION was appliedon the surface of the samples (i.e. coatings of the solder mask inkjetink on a copper surface) to clean the surface. The solder flux was driedby placing the samples for 10 minutes above the solder pot.

After placing the sample in the solder pot, a solder wave was createdfor 10 seconds after which the samples were cooled for at least 10minutes.

The adhesion of the solder mask inkjet inks on the copper surface wasthen evaluated with a tape test on the cooled samples. A black tape Tesa4104/04 from TESA AG, Germany was taped onto the coating and the tapewas removed immediately thereafter by hand.

A visual evaluation resulted in an adhesion quality ranging from 0 (verygood adhesion) to 5 (very poor adhesion).

Adhesion

The adhesion was evaluated by a cross-cut test according toISO2409:1992(E). Paints (International standard 1992-08-15) using aBraive No. 1536 Cross Cut Tester from BRAIVE INSTRUMENTS with spacing ofa 1 mm between cuts and using a weight of 600 g, in combination with aTesatape™ 4104 PVC tape. The evaluation was made in accordance with acriterion described in Table 3, where both the adhesion in the cross-cutand outside the cross-cut were evaluated.

TABLE 3 Evaluation value Criterion 0 Nothing removed, perfect adhesion.1 Detachment of only very small parts of the cured layer, almost perfectadhesion. 2 Minor parts of the cured layer was removed by the tape, goodadhesion 3 Parts of the cured layer were removed by the tape, pooradhesion. 4 Most of the cured layer was removed by the tape, pooradhesion. 5 The cured layer was completely removed from the substrate bythe tape, no adhesion.Viscosity

The viscosity of the inks was measured at 45° C. and at a shear rate of1 000 s⁻¹ using a “Robotic Viscometer Type VISCObot” from CAMBRIDGEAPPLIED SYSTEMS.

For industrial inkjet printing, the viscosity at 45° C. and at a shearrate of 1000 s⁻¹ is preferably between 3 and 20 mPa·s. More preferablythe viscosity at 45° C. and at a shear rate of 1000 s⁻¹ is less than 15mPa·s.

Preparation of the Adhesion Promoters

The Synthesis of THIO-COMP

15 g (0.1 mol) 2,5-dimercapto-[1,3,4]-thiadiazole was dissolved in 70 mlacetone. 10.1 g (0.1 mol) triethyl amine and 0.5 g sodium iodide wereadded. 15 g (0.083 mol) 2-bromo-propionic acid ethyl ester was added andthe mixture was stirred for one hour at 25° C. 500 ml water was added.The separated organic layer was isolated. 200 ml methylene chloride wasadded and the solution was extracted once with 200 ml water and oncewith 200 ml of a 1M NaHCO₃ solution. The organic layer was isolated,dried over MgSO₄ and evaporated under reduced pressure. 12 g (yield=57%)of 2-(5-mercapto-[1,3,4]thiadiazol-2-ylsulfanyl)-propionic acid ethylester was isolated as an oily compound (TLC analysis on Partisil KC18Fsupplied by Whatman using MeOH/0.5 M NaCl as eluent: R_(f)=0.53).

The Synthesis of THIO-1

375.6 g (2.5 mol) 2,5-dimercapto-[1,3,4]-thiadiazole was suspended in1.5 liter water. 500 ml of a 5N sodium hydroxide solution was added andthe mixture was stirred for 15 minutes. The mixture was heated to 75° C.and 401.6 g (2.625 mol) 2-bromo-propionic acid was added over one and ahalf hour. The reaction mixture was stirred for an additional 15 minutesand the mixture was cooled to room temperature. Thio-1 precipitated fromthe medium, was isolated by filtration, washed with 500 ml water anddried. 524.4 g (y: 94.4%) of Thio-1 was isolated (TLC analysis on aSilicagel 60 F254 TLC plate supplied by Merck using methylenechloride/methanol/acetic acid 80/20/10: R_(f)=0.86).

The Synthesis of THIO-2

6 g (40 mmol) 2,5-dimercapto-[1,3,4]-thiadiazole was dissolved in asolution of 4.24 g (40 mmol) sodium carbonate in 100 ml water. Asolution of 3.94 g (20 mmol) bromo-succinic acid and 4.24 g (40 mmol)sodium carbonate in 100 ml water was added and the reaction was allowedto continue for 24 hours at room temperature. The mixture was refluxedfor an additional half an hour. The mixture was allowed to cool down toroom temperature and the mixture was acidified to pH=1 using a 5Nhydrochloric acid solution. The mixture was extracted with 100 mlmethylene chloride. The aqueous phase was isolated and extracted with100 ml ethyl acetate. The ethyl acetate was dried over MgSO₄ andevaporated under reduced pressure. 3.72 g (yield=70%) of Thio-2 wasisolated. The structure of Thio-2 was confirmed using ¹H-NMR analysis.

Example 1

This example illustrates the superior solder mask resistance of UVcurable inkjet inks including adhesion promoters according to thepresent invention.

Cyan and Yellow Pigment Dispersions CPD and YPD

Concentrated Cyan and Yellow and pigment dispersions, respectively CPDand YPD, were prepared having a composition according to Table 4.

TABLE 4 wt % of: CPD YPD Cyan 15 — Yellow — 15 Disperbyk 162 15 = INHIB 1 = VEEA 69 =

CPD and YPD were prepared as follows: 138 g of 2-(2-vinyloxyethoxy)ethylacrylate, 2 g of a solution containing 4 wt % of 4-methoxyphenol, 10 wt% of 2,6-Di-tert-butyl-4-methylphenol and 3.6 wt % of Aluminum-N-nitrosophenylhydroxyl amine in dipropylene glycol diacrylate, 200 g of a 30 wt% solution of Disperbyk 162 in 2-(2-Vinyloxyethoxy)ethyl acrylate and 60g of Cyan (for CPD) or 60 g of Yellow (for YPD) were mixed using aDISPERLUX™ dispenser. Stirring was continued for 30 minutes. The vesselwas connected to a NETZSCH MiniZeta mill filled with 900 g of 0.4 mmyttrium stabilized zirconia beads (“high wear resistant zirconiagrinding media” from TOSOH Co.). The mixture was circulated over themill for 120 minutes (residence time of 45 minutes) and a rotation speedin the mill of about 10.4 m/s. During the complete milling procedure thecontent in the mill was cooled to keep the temperature below 60° C.After milling, the dispersion was discharged into a vessel.

The resulting concentrated pigment dispersions CPD and YPD exhibited anaverage particle size of respectively 80 nm and 131 nm, as measured witha Malvern™ nano-S, and a viscosity of respectively 51 mPa·s and 114mPa·s at 25° C. and at a shear rate of 10 s⁻¹.

Preparation of Comparative Ink COMP-1 and Inventive Inks INV-1 to INV-7

The comparative radiation curable inkjet ink COMP-1 and the inventiveradiation curable inkjet ink INV-1 to INV-7 were prepared according toTable 5. The weight percentages (wt %) are all based on the total weightof the radiation curable inkjet ink.

TABLE 5 wt % of component COMP-1 COMP-2 INV-1 INV-2 INV-3 INV-4 INV-5CPD 6.65 = = = = = = YPD 6.65 = = = = = = SR606A 33.00 = = = = = = TMPTA5.00 = = = = = = VEEA 36.45 31.45 = = = = = ITX 4.00 = = = = = = TPO2.95 = = = = = = EPD 4.00 = = = = = = THIO-COMP — 5.00 — — — — — THIO-01— — 5.00 — — — — THIO-02 — — — 5.00 — — — THIO-03 — — — 5.00 — — THIO-06— — — — — 5.00 — THIO-09 — — — — — — 5.00 Ebecryl 1360 AK 0.10 = = = = == INHIB 1.00 = = = = = =

The solder resistance of the Comparative ink COMP-01 and the inventiveinks INV-01 to INV-05 were tested as described above. The results areshown in Table 6.

TABLE 6 UV curable ink jet ink Adhesion Solder Resistance COMP-1 5 5COMP-2 5 5 INV-1 0 0 INV-2 0 0 INV-3 0 0 INV-4 0 0 INV-5 1 0

It is clear from the results of Table 6 that the inventive solder maskinkjet inks containing an adhesion promoter according to the presentinvention all have an improved adhesion towards the Cu-surface and asuperior solder resistance compared to a solder mask inkjet ink withoutsuch an adhesion promoter.

The invention claimed is:
 1. A radiation curable solder mask inkjet inkcomprising: a photoinitiator; a radical polymerizable compound; and anadhesion promoter; wherein the adhesion promoter has a chemical formulaaccording to Formula II:

wherein L₂ represents an optionally substituted o+p-valent linking groupincluding 1 to 10 carbon atoms; o and p independently represent aninteger from 1 to 4; and Q represents atoms that form an optionallysubstituted 5 or 6 membered heteroaromatic ring.
 2. The radiationcurable solder mask inkjet ink according to claim 1, wherein Qrepresents atoms that form an optionally substituted 5 memberedheteroaromatic ring selected from the group consisting of a1,2,3-triazole, a 1,2,4-triazole, a 1,3,4-oxadiazole, a1,3,4-thiadiazole, a tetrazole, an imidazole, a benzimidazole, anoxazole, a benzoxazole, a thiazole, and a benzthiazole.
 3. The radiationcurable solder mask inkjet ink according claim 1, wherein the radicalpolymerizable compound is selected from the group consisting ofneopentyl glycol hydroxypivalate diacrylate, isobornyl acrylate,dipropylene glycol diacrylate, trimethylol propane triacrylate, and2-(vinylethoxy)ethyl acrylate.
 4. The radiation curable solder maskinkjet ink according to claim 1, wherein an amount of the adhesionpromoter is between 0.5 and 20 wt % relative to a total weight of thesolder mask inkjet ink.
 5. The radiation curable solder mask inkjet inkaccording to claim 1, further comprising a cyan pigment, a yellowpigment, or a green pigment.
 6. A method of manufacturing an electronicdevice, the method comprising: jetting a radiation curable solder maskinkjet ink as defined in claim 1 onto a dielectric substrate includingan electrically conductive pattern; and curing the jetted solder maskinkjet ink.
 7. The method according to claim 6, wherein the step ofcuring is performed using UV radiation.
 8. The method according to claim6, further comprising: heating the jetted solder mask inkjet ink.
 9. Themethod according to claim 8, wherein the step of heating step isperformed at a temperature between 80° C. and 250° C.
 10. The methodaccording to claim 9, wherein the step of curing is performed using UVLED radiation.
 11. The method according to claim 6, wherein theelectrically conductive pattern includes copper.
 12. The methodaccording to claim 6, wherein the electronic device is a printed circuitboard.